Particle acceleration tube having improved beam focus control

ABSTRACT

Improved focusing of a beam of charged particles is achieved in an acceleration tube comprising alternate insulating rings and apertured electrodes to which progressively increasing electric potentials are applied to create an electric field for accelerating the charged particles along the length of the tube. In order to achieve precise beam focusing for different electric field strengths the aperture of the grounded electrode at the tube entrance is covered with a grid-like structure and a variable control voltage is applied to a downstream control electrode. For different field strengths the control voltage is adjusted to modify the field between the grounded and control electrodes to provide a variable lens effect. A preferred embodiment employs a variable resistor connected between the grounded and control electrodes to provide the variable control voltage.

Unite States atent [191 [111 3,73t,2ii Purser May 1, 1973 [54] PARTICLE ACCELERATION TUBE HAVING IMPROVED BEAM FOCUS CONTROL Inventor: Kenneth H. Purser, Honeoye, N.Y.

Assignee: Radiation Dynamics, Inc., Westbury,

Sept. 29, 1969 Filed:

Appl. No.:

Purser ..3l3/63 X Mobley ..328/233 Primary Examiner-Roy Lake Assistant ExaminerPalmer C. Demeo Attorney-Rose & Edell [57] ABSTRACT Improved focusing of a beam of charged particles is achieved in an acceleration tube comprising alternate insulating rings and apertured electrodes to which progressively increasing electric potentials are applied to create an electric field for accelerating the charged particles along the length of the tube. In order to achieve precise beam focusing for different electric field strengths the aperture of the grounded electrode at the tube entrance is covered with a grid-like structure and a variable control voltage is applied to a downstream control electrode. For different field strengths the control voltage is adjusted to modify the field between the grounded and control electrodes to provide a variable lens effect. A preferred embodiment employs a variable resistor connected between the grounded and control electrodes to provide the variable control voltage.

10 Claims, 9 Drawing Figures l l LENS g PATENTEDHAY 1 191a SHEET 1 OF 2 H\GH VOLTAGE SUPPLY N SABRCE INVENTOR KENNETH HPURSER ATTORNEYS PARTICLE ACCELERATION TUBE HAVING IMPROVED BEAM FOCUS CONTROL BACKGROUND OF THE INVENTION The present invention relates to charged particle accelerators, and more particularly to focusing beams of charged particles injected into acceleration tubes.

It is unfortunately true that the trajectories of particles leaving an ion source are neither exactly parallel nor do they originate from a single point on the axis of the source. Because of this, it is usually necessary to incorporate, between the ion source and the target, some form of focusing device to handle beams of such ions and to compress the beam radially so that particles will not be lost at various defining apertures.

In accelerators such as the Van de Graaff type, which operate by allowing ions to fall through nearly uniform dc electric fields, important focusing is usually produced at the entrance and exit from the accelerating region by the radial components of the bulging distribution of equipotential field surfaces. In practice, the strongest lens effects usually occur at the entrance to the acceleration region as it is here that the ion velocities are low and consequently the radial components of the electric field have the longest time to operate.

If the particle transmission efficiency of such an accelerator is to be high over a wide range of acceleration energies, it is usually essential that the optical characteristics of the machine remain approximately constant. The characteristics which are particularly important include the location of crossovers, the relative beam diameter between successive crossovers, the principal planes of lenses, etc. This constancy of optical behavior with energy is not easily achieved for different acceleration energies when the energy of the ions from the ion source remains constant. The reason is that the focal strength of the accelerator as a whole and, most important, the focal length of the lenses at the entrance and exit to the acceleration region change with the accelerator terminal voltage which creates the acceleration-inducing electrostatic fields, the lens strength being greatest when the terminal potential is highest, and weakest when the terminal potential is lowest.

One solution to this serious problem of changing focal lengths is to vary the energy of the ions from the source linearly with the terminal voltage of the accelerator. Although this technique can be made to work well, the effect is merely to transfer the difficulties from the accelerator to the ion source. As an example, if the range of ion energies from the accelerator is to vary by a factor of 4:1, the injection energy has to change in the same ratio. As the minimum extraction energy is on the order of 50 keV, this solution requires expensive power supplies and introduces insulation difficulties. Also, for machines using negative ions the charge exchange efficiencies are no longer constant unless elaborate optics are employed.

The most satisfactory suggestion that has been made to minimize the changing optical effects of the accelerator is to terminate the uniform field region of the accelerator by a flat or planar metallic grid having high transparency for charged particles. This provides a definite flat or planar termination rather than the bulging termination in free space. The effect is that the electrostatic field within the acceleration region remains uniform throughout the critical volume of the acceleration region and eliminates completely the variable radial field components at the entrance to the accelerator which vary with terminal voltage.

While this solution satisfies the requirement of constancy of focal lengths, the arrangement has proven to be too extreme. Some lens action is almost always needed close to the entrance to the accelerator to focus the beam toward the target or, in the case of the tandem accelerator, through the terminal stripping canal. In the past, when the acceleration tube has been gridded as described in the previous paragraph an additional focusing element has had to be provided. This has been accomplished as described in my prior U. S. Pat. No. 3,423,684 by electrically isolating the entrance to the acceleration region from the terminal voltage source and by elevating this entrance point, defined by the planar grid, in electrical potential to cause a focusing effect between the grid and a cylindrical bipotential lens. To be effective, a substantial power supply is needed which must be capable of supplying many tens of kilovolts and operating in a rather hostile environment in which it is difficult to avoid large high voltage surges in the supply when the accelerator sparks. In addition, this arrangement requires that both the power supply for the cylindrical bipotential lens and the control voltage applied to the gridded electrode be adjusted each time the accelerator tube terminal voltage is changed in order to optimize beam focusing. Further, for reasons that are described in detail below, this arrangement requires a reversible polarity control voltage power supply to achieve both convergent and divergent lens effects in the acceleration tube. Reversible polarity supplies are relatively expensive and complex. Clearly then the prior art focusing arrangement could use some improvement.

It is therefore an object of the present invention to provide an arrangement which permits precise focusing of a charged particle beam in an accelerator.

It is another object of the present invention to achieve precise focusing of a charged particle beam in an accelerator tube over a wide range of accelerator terminal voltages.

It is still another object of the present invention to minimize radial field components at the entrance to the acceleration region of a charged particle accelerator.

It is still another object of the present invention to provide an improved apparatus and method of keeping the focal length in an acceleration tube substantially constant over the whole energy range of the tube without completely eliminating focal properties of the tube.

It is still another object of the present invention to achieve both convergent and divergent lens effects in a particle acceleration tube without employing a reversible polarity power supply.

SUMMARY OF THE INVENTION In accordance with the principles of the present invention, the electrode at the entrance to the accelerator has its aperture covered with a grid. In addition, a variable control voltage is applied to a control electrode located downstream of the gridded electrode. The control voltage determines the strength of the lens thusly created downstream of the grid, the strength of this lens alone being adjustable to the value required to focus ions to the target or, in the case of tandem accelerators, through the stripping canal. No other auxiliary focusing element is needed since the lens strength is set at the value needed for focusing rather than to zero strength as is done in the simple gridded electrode arrangement. The present approach does away with the requirement for a high voltage supply for control purposes since a simple variable resistor may be connected between the gridded and control electrodes to effect lens strength control. In addition, the gridded electrode, which is maintained at either the high or low terminal voltage of the tube, is located at the acceleration tube entrance, assuring a planar rather than a bulging field termination at said entrance; this eliminates the uncontrolled lens effect at the tube entrance. A further advantage of the present invention resides in the fact that since the gridded electrode is grounded there is less sparking and metal sputtering than is the case in my U. S. Pat. No. 3,423,684 wherein the grid is at some relatively high potential. This improves the life of the grid, avoids deposition of metal throughout the accelerating structure,-and improves the reliability of the high voltage circuitry since the high voltage lens supply is not affected by tube surges.

BRIEF DESCRIPTION OF THE DRAWINGS The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagrammatic illustration of an electrostatic accelerator of the tandem type;

FIG. 2 is a plan view in partial section of the entrance end of accelerator of FIG. 1, illustrating the features of the present invention;

FIG. 3 is a schematic illustration of the features of the present invention;

FIGS. 4a, 4b and 4c are diagrammatic illustrations of focusing effects achievable in a prior art accelerator;

FIGS. 5a, 5b and 5c are diagrammatic illustrations of focusing effects achievable by the arrangement of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now specifically to FIG. 1 of the accompanying drawings there is illustrated a tandem-type particle accelerator having a grounded casing 11 and high voltage terminal 13 supported substantially at the longitudinal center of casing 11. High voltage is applied across terminal 13 and casing 11, plus to minus respectively, from a high voltage supply 14, for example of the type disclosed in U. S. Pat. No. 3,178,604. Evacuated acceleration tubes 15 and 17 are supported within casing 11, extending longitudinally therein from respective casing ends to high voltage terminal 13. Tubes 15 and 17 are aligned with one another and with stripping element 19 located within terminal 13. Negative ions are emitted from an ion source 21 and injected in a beam into acceleration tube 15 within which they are accelerated toward high voltage terminal 13. Electrons are removed from the negative ions in stripping element l9, and the positive ions thus formed are acceierated through tube 17 to the other end of the accelerator 10.

By way of example only, stripper element 19 may be of the type disclosed in the article A High Efficiency Ion Optical System For Tandem Accelerators by Brooks, et al., appearing in the June, 1965 issue of I.E.E.E. Transactions On Nuclear Science, pages 313 through 3 If); however any conventional electron stripping unit may be employed.

It is generally a requirement that the energy of the ion beam be variable over a specified range. This is done by varying the terminal voltage, V applied by supply 14 across terminal 13 and casing 11. This correspondingly varies the voltage gradient in the electric fields which appear across both acceleration tubes 15 and 17.

Referring now to FIGS. 2 and 3 of the accompanying drawings there are illustrated, respectively, a partial section and a schematic diagram of the upstream end of acceleration tube 15 of FIG. 1. By upstream end is meant the end through which the ion beam enters tube 15. As illustrated, tube 15 comprises a series of alternating apertured electrodes 23 and insulating rings 25. The apertures 27 in the electrodes are axially aligned to define a path for the ion beam through the tube 15. The extreme upstream electrode is at casing potential (ground) and has its aperture covered bya grid 28 comprising a metallic mesh screen.

The terminal voltage V, applied between terminal 13 and casing 11 is subdivided by a resistor column or string 29. Successive electrodes 23 are connected to successive points along resistor string 29 to define a uniform electric field across the length of tube 15.

A variable resistor 31 is connected between the extreme upstream electrode 23' and a predetermined one of electrodes 23, the latter being referred to herein as control electrode 23". In FIGS. 2 and 3 control electrode 23" is illustrated as the fourth electrode from the upstream end of tube 15; however, this is not a limiting factor since virtually any of the un-gridded electrodes 23 which are near the entrance to the tube 15, could serve as the control electrode. The function of variable resistor 31 is to permit selective variation of the electric field between control electrode 23" and gridded electrode 23. As illustrated in FIG. 3, variable resistor 31 may be replaced by a variable voltage supply 33; however variable resistor 31 is utilized in the preferred embodiment of the present invention. In addition, an einzel lens 35 (illustrated in FIG. 3) controlled by a lens voltage supply 37 may optionally be employed upstream of gridded electrode 23" to provide auxiliary focusing of the ion beam. It must be stressed that this latter feature is entirely optional; the considerations involved in the employment of lens 35 are discussed in detail below.

The various electrodes 23 define equipotential surfaces in the electric field created along the length of tube 7. In prior art devices, lacking grid'28 and variable resistor 31, these equipotential surfaces are substantially planar, except at the upstream end of tube 15 where the surfaces of equipotential distribution tend to bulge outwardly in an upstream direction. This phenomenon is described and illustrated in the abovereferenced Brooks, et al. article, which also discloses that the provision of a grounded grid, such as grid 28, at

the upstream end of the acceleration tube eliminates the bulging equipotential distribution by terminating the electric field in a planar surface. It is the bulging field which creates a lens effect on the ion beam injected into the acceleration tube. Since the extent of the bulge depends on the terminal voltage, V focusing of the beam at stripper element 19 would vary with terminal voltage unless planar grid 28 is provided. My above-referenced U. S. Pat. No. 3,423,684 discloses an arrangement which includes a planar grid and discusses a problem encountered thereby, namely that the grid, of itself, eliminates all focusing, raising a need for an additional focusing control to assure that the beam is focused at the stripper element. Said patent consequently employs an einzel or bipotential lens upstream of the entrance to the acceleration tube to provide focusing control independently of the tube terminal voltage. The einzel or bipotential lens focuses the beam to provide a point object at the proper distance upstream of the acceleration tube. However, the beam diverges from the point object and a corrective convergent lens must be provided close to the entrance of the tube in order to obtain a point image at the stripper element. The technique for achieving the convergent lens in my aforementioned patent is to move the grid to an electrode downstream of the grounded electrode and to apply a control voltage to the gridded electrode. This technique however has its own disadvantages, such as requireing an additional high voltage supply for the control voltage, requiring individual control over both the einzel and convergent lenses, and in some cases causing the maximum voltage rating of the accelerator to be exceeded for reasons discussed below. These disadvantages, as well as how they are overcome in the present invention, are best understood upon consideration of FIGS. 4a, 4b, 40, 5a, 5b and 5c.

In FIG. 4a there is diagrammatically illustrated a prior art device of the type disclosed in my abovereferenced U. S. Pat. No. 3,423,684. The illustration includes the upstream end of the accelerator tube with extreme upstream electrode 43'- grounded and a grid 48 covering the aperture of a control electrode 43" comprising the fourth electrode 43 from the upstream end of the tube. In FIG. 4a zero volts are applied to control electrode 43", rendering the entire region between electrodes 43' and 43" equipotential. The surfaces of equipotential distribution, indicated as lines in FIG. 4a, are planar downstream of grid 48 and no bulging is evident. There is thus no effective lens provided at the upstream end of the tube, and the forces acting on the ions because of the field are parallel to the beam direction as indicated by the arrows in FIG. 4a.

In FIG. 4b the same prior art device is illustrated with a positive voltage +V applied to control electrode 43". The lines of equipotential distribution are seen to remain planar downstream of grid 48; however on the upstream side of the grid the equipotential lines bulge outwardly in an upstream direction, the extent of the bulge being determined by the magnitude of +V,. The effect of this bulging is to converge the beam as it passes through this region. The reason for this is that the forces exerted by the field on the negative ions are in the direction of increasing potential (minus to plus) and perpendicular to the equipotential surfaces, as indicated by the arrows in FIG. 4b.

In FIG. 40 the same prior art device is illustrated with a negative control voltage V,, applied to control electrode 43". The lines of equipotential distribution again are planar downstream of grid 48; and upstream of grid 48 these lines again bulge outwardly in an upstream direction. However, the forces acting on the ions in the upstream section of the tube are directed oppositely to those in FIG. 4b, as indicated by the arrows in FIG. 4c. The reason for this is that the negative ions traverse a decreasing field (ground to minus) in this section of the tube. The effect of this bulging is to provide a divergent lens for the ion beam passing therethrough.

Referring now to FIG. 5a, the focusing effect of the arrangement of the present invention is illustrated when a control voltage V in the range 0 s V (nV,/N) is applied to control electrode 23". Here, N is the total number of electrodes 43 in tube 15 and n is the number of spacers 25 separating electrode 43" from electrode 43. The lines of equipotential distribution are planar at the grounded and gridded electrode 23'; however downstream of grid 28 these lines bulge outwardly in an upstream direction, the bulge being substantially centered about control electrode 23". The bulge is maximum when V is zero and becomes smaller as V approaches the constant gradient value V=(nV,/N) The effect of the forces exerted by this equipotential distribution is to converge the beam as indicated by the arrows in FIG. 5a.

As illustrated in FIG. 5b, when the control voltage applied to control electrode 23" is equal to (nV,/N), there is zero lens effect at the entrance to the tube because all the equipotential lines are planar. This result is also borne out mathematically by the wellknown Davisson formula for a single aperture lens, to wit:

f I/( 2' i) l. where f is the focal length of the lens; E is the electric field strength downstream of the lens element; E 1 is the electric field strength upstream of the lens element; and V is the voltage through which an ion falls before passing through the lens. The focal length is seen to approach infinity (zero lens strength) when E, E,. When the control voltage V is equal to (nV,/N), then E E and zero lens strength is provided. The Davisson formula, equation 1, provides a fairly good approximation of the focal length of the lens created by the control voltage applied to control electrode 23".

In FIG. 5c the arrangement of the present invention is illustrated wherein the control voltage applied to control electrode 23" is greater than (nVJN). Under such circumstances the lines of equipotential distribution bulge oppositely to the lines in FIG. 5a and to an extent which is dependent upon the magnitude of the control voltage V. The effect, as indicated by the arrows in FIG. 50, is to provide a divergent lens for the negative ion beam.

An important advantage of the present invention is apparent from a comparison between FIGS. 4a, b, c, and FIGS. 5a, b, c. More specifically, in the present invention one is able to obtain both convergent and divergent lens effects with a control voltage of one polarity. In the prior art device, illustrated in FIGS. 4a, b, c, convergent and divergent lens effects are achieved with different control voltage polarities. The prior art device thus requires a substantially more complex control voltage supply than the present invention. More importantly, the present invention can utilize a simple and reliable variable resistor 31 to provide the control voltage'whereas the prior art device requires a relatively expensive high voltage supply which tends to be unreliable and subject to damage by surges.

Another advantage of the present invention over my prior art arrangement arises from the fact that the grid 28 in the present invention is grounded whereas in the prior art arrangement the grid is at some control potential. In both cases the grid intercepts some beam current. In the prior art arrangement such beam current interception loads the control voltage supply significantly. Even more important, when the accelerator sparks the supply is subject to damage due to current surges. In the present invention there is no control supply loading caused by beam current interception and high current discharges do not terminate at the power supply.

In addition, application of a negative control voltage to control electrode 43" in FIG. 40 renders the voltage drop between high voltage terminal 13 (FIG. 1) and control electrode 43"in excess of the terminal voltage V,. If the accelerator is operating at maximum rated terminal voltage, a negative control voltage produces a total voltage exceeding the maximum rating of the tube, resulting in sparking and rapid deterioration of the tube. In the present invention the variable resistor 31 or unipolar variable voltage supply 33 maintain the voltage between control terminal 23" and gridded terminal 23' at or below the voltage drop provided therebetween by resistor string 29 in the absence of a control voltage.

Another advantage of the present invention resides in the fact that nothing but the lens provided by virtue of the control voltage applied to control electrode 23" is required to achieve beam focusing; simple adjustment of resistor 31 selectively changes the optical properties of the tube and is all that is required for refocusing.

As indicated above, and referring to FIG. 3, einzel lens 35 maybe employed in conjunction with the arrangement of the present invention. Since the lens achieved by virtue of the control voltage applied to control electrode 23" is readily capable of becoming divergent, the convergence provided by the einzel lens 35 in combination with a divergent control lens offers the possibility of a wider range of focusing.

While 1 have described and illustrated specific em bodiments of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. A device for accelerating ions injected thereinto in a downstream direction, said device comprising an acceleration tube having a series of alternating insulating rings and apertured electrodes, means for applying successive voltage increments across adjacent electrodes to provide a substantially uniform electric field in said acceleration tube, an electrically conductive grid-like structure covering the aperture of one of said electrodes, and means for applying a control voltage to a second electrode disposed downstream of said one electrode to render the electric field between said one and said second electrodes non-uniform.

2. A device for accelerating charged particles comprising:

an acceleration tube having an upstream end and a downstream end and comprising a multiplicity of alternating apertured electrodes and insulating rings, the apertures of said electrode being axially aligned to define a path for said charged particles therethrough between the upstream and downstream ends of said tube;

first means for applying successive voltage increments across adjacent electrodes to provide an electric field between the upstream and downstream ends of said tube;

second means for providing a flat planar termination for said electric field proximate the upstream end of said tube;

third means for injecting a beam of said charged particles into said tube at the upstream end thereof and directed generally toward the downstream end thereof; and

focus control means for permitting said beam to be focused toward the downstream end of said tube, said focus control means comprising fourth means for selectively varying the potential at a control electrode, said control electrode comprising one of said apertured electrodes disposed downstream of said planar termination.

3. The device according to claim 2 wherein said fourth means comprises means for selectively varying the resistance between said control electrode and an electrode proximate the upstream end of said tube.

4. The device according to claim 2 wherein said second means comprises an electrically conductive grid-like structure covering the aperture of the extreme upstream electrode and wherein said fourth means comprises a variable resistor connected between said control electrode and said extreme upstream electrode.

5. The device according to claim 4 wherein said charged particles are negative ions, wherein said successive voltage increments are arranged plus to minus in an upstream direction, and wherein said extreme upstream electrode is at electrical ground potential.

6. The device according to claim 5 wherein said tube is part of a tandem-type accelerator further comprising an electron stripper element disposed at the downstream end of said tube and toward which said beam is focused.

7. The device according to claim 2 wherein said fourth means is capable of selectively providing convergent and divergent refraction of said beam by varying the potential at said control electrode over a unipolar potential range, said device further comprising additional convergent lens means disposed upstream of the upstream end of said tube for converging said beam prior to its injection into said tube.

8. The device according to claim 2 wherein said second means comprises an electrically conductive grid-like structure covering the-aperture of one of said electrodes proximate the upstream end of said tube.

1 9. The device according to claim 8 wherein said fourth means comprises a unipolar variable voltage supply connected to said control electrode. 

1. A device for accelerating ions injected thereinto in a downstream direction, said device comprising an acceleration tube having a series of alternating insulating rings and apertured electrodes, means for applying successive voltage increments across adjacent electrodes to provide a substantially uniform electric field in said acceleration tube, an electrically conductive grid-like structure covering the aperture of one of said electrodes, and means for applying a control voltage to a second electrode disposed downstream of said one electrode to render the electric field between said one and said second electrodes non-uniform.
 2. A device for accelerating charged particles comprising: an acceleration tube having an upstream end and a downstream end and comprising a multiplicity of alternating apertured electrodes and insulating rings, the apertures of said electrode being axially aligned to define a path for said charged particles therethrough between the upstream and downstream ends of said tube; first means for applying successive voltage increments across adjacent electrodes to provide an electric field between the upstream and downstream ends of said tube; second means for providing a flat planar termination for said electric field proximate the upstream end of said tube; third means for injecting a beam of said charged particles into said tube at the upstream end thereof and directed generally toward the downstream end thereof; and focus control means for permitting said beam to be focused toward the downstream end of said tube, said focus control means comprising fourth means for selectively varying the potential at a control electrode, said control electrode comprising one of said apertured electrodes disposed downstream of said planar termination.
 3. The device according to claim 2 wherein said fourth means comprises means for selectively varying the resistance between said control electrode and an electrode proximate the upstream end of said tube.
 4. The device according to claim 2 wherein said second means comprises an electrically conductive grid-like structure covering the aperture of the extreme upstream electrode and wherein said fourth means comprises a variable resistor connected between said control electrode and said extreme upstream electrode.
 5. The device according to claim 4 wherein said charged particles are negative ions, wherein said successive voltage increments are arranged plus to minus in an upstream direction, and wherein said extreme upstream electrode is at electrical ground potential.
 6. The device according to claim 5 wherein said tube is part of a tandem-type accelerator further comprising an electron stripper element disposed at the downstream end of said tube and toward which said beam is focused.
 7. The device according to claim 2 wherein said fourth means is capable of selectIvely providing convergent and divergent refraction of said beam by varying the potential at said control electrode over a unipolar potential range, said device further comprising additional convergent lens means disposed upstream of the upstream end of said tube for converging said beam prior to its injection into said tube.
 8. The device according to claim 2 wherein said second means comprises an electrically conductive grid-like structure covering the aperture of one of said electrodes proximate the upstream end of said tube.
 9. The device according to claim 8 wherein said fourth means comprises a unipolar variable voltage supply connected to said control electrode.
 10. The device according to claim 8 wherein said fourth means comprises a variable resistor connected between said control electrode and said one of said electrodes. 